1. Field of the Invention
This invention relates to electrochromic (EC) devices which can vary the transmission or reflectance of electromagnetic radiation by application of an electrical stimulus to the EC device. In particular, the EC devices of this invention employ a selective ion transport layer in combination with an electrolyte containing at least one redox active material to provide excellent device response and stability.
2. Related Prior Art
Electrochromic devices are well known. Such devices are described, for example, in U.S. Pat. No. 4,435,408 (an electrochromic device comprising a pair of transmissive electrodes having disposed therebetween an electrochromic material and an electrolyte layer containing a non-liquid material of an adhesive or tacky high polymer and a material having a coordinating function); GB2268595 (an electrochromic device comprised of a substrate bearing an electrode followed by an electrochromic layer, an electrolyte layer containing a cation doner material and a counter electrode on a second substrate); JP 63-106731 (an EC device comprising mutually opposing electrode plates having therebetween an electrochromic material layer and an electrolyte consisting of either a lactone organic solvent or an organic solvent containing S=0, a cation source, a polymer for gelling the electrolyte and at least one kind of ferrocene compound acting as a redox agent) and JP 63-106730 (an EC device comprising mutually opposing electrode plates having therebetween an electrochromic material layer and an electrolyte including an organic solvent having S=0, a quinone compound and/or a ferrocene compound as a redox agent and an iodine compound).
There are many potentially useful applications for EC devices. For example, they may be employed in glazings, e.g., energy efficient and/or privacy windows for architectural or automotive use, automotive rearview mirrors, displays, filters and other applications where variable light transmission is desired.
A typical prior art window device is illustrated in FIG. 1 having substrates (e.g., glass) 10 and 15, transparent conductors 20 and 25 (e.g., indium tin oxide or doped tin oxide), electrochromic layer 30 (e.g., tungsten oxide, molybdenum oxide or polyaniline), and an electrolyte 40. The electrolyte layer may consist of at least one redox active species and at least one dissociable acid, lithium salt or the like, such as disclosed, for example, in copending U.S. application Ser. No. 08/547,678, entitled "Electrochromic Devices", filed Oct. 24, 1995, and copending U.S. application Ser. No. 08/330,090, entitled "Electrochromic Device with Improved Processability and Methods of Preparing the Same", filed Oct. 26, 1994.
As noted above, electrochromic devices of the prior art undergo a change in electromagnetic radiation transmission after the application of an electrical stimulus. For example, if the EC device illustrated in FIG. 1 used tungsten oxide as an electrochromic layer 30, then when an appropriate potential is applied to the device, the redox material in the electrolyte 40 would be oxidized at the electrolyte 40/transparent conductor 25 interface which would cause a simultaneous reduction in the tungsten oxide by the injection of ions and electrons into the electrochromic layer 30. However, the oxidized species can typically migrate to the electrolyte 40/tungsten oxide 30 interface and undergo a reduction to its original form, thus resulting in a steady back reaction.
As a result of this steady back reaction, typically an EC device must remain exposed to the electrical potential used to reach a desired transmissive state in order to maintain that state of transmission. If the electrical stimulus is removed, then generally over time the EC device will return to the transmissive state it held prior to application of the electrical stimulus, i.e., the rest state. Such devices are said to have a short memory. Indeed, if the back reaction is significant, then with increasing area of the device it is more difficult to maintain a uniform transmissive state, e.g., uniform coloration, over the entire area of such a device.
While some applications may benefit from a large back reaction in electrochromic devices, such as rearview automotive mirrors, other applications may require reduced or no back reaction in the low transmission state. For example, EC devices employed as architectural or automotive glazings, e.g., automotive sunroofs, would benefit from reduced back reaction since a glazing could be left in a low transmission or darkened state without the need for constant voltage application.
Several techniques have been adopted in an attempt to enhance the memory of EC devices. For example, Anderson, A. M., et al., "An Electrochromic Li.sub.x Wo.sub.3 /polymerlaminate/Li.sub.y V.sub.2 O.sub.5 Device", Large-Area Chromogenic: Materials and Devices for Transmittance Control, SPIE Institute Series Vol. IS4, pp. 471-481 (1990), describes an electrochromic window device comprised of two pieces of conductive glass with complementary electrochromic coatings connected by a transparent polymeric ion conductor. This publication describes EC devices where an additional coating, i.e., a "counter coating" is deposited on a transparent conductor, such as transparent conductor 25 shown in FIG. 1. This counter coating is in contact with the electrolyte. However, the electrolyte is only an ion conducting medium and contains no redox active species. Moreover, the EC device of this publication requires that either the electrochromic layer or the counter coating layer must be pre-reduced, i.e., ionic intercalated, to a specific degree to control the device contrast in a desired range. In addition, the two coatings, i.e., the electrochromic coating and the counter coating, have to be electrochemically matched for the desired performance. Because of these requirements, the assembly of such EC devices is generally cumbersome and expensive.
A device that remains in a low transmissive state in the rest state is disclosed in copending application Ser. No. 08/547,578, entitled "Electrochromic Devices", filed Oct. 24, 1995. However, this device utilizes the continuous application of electrical power to the device to remain in the high transmissive state, e.g., the bleached stated.
JP 5-80357 discloses an electrochromic device allegedly having an improved memory comprising two opposing electrodes having disposed therebetween an electrochromic material layer and an electrolyte layer separated by an ion conducting barrier layer. The ion conducting layer is preferably a metal, such as a metal oxide or metal halide. This publication does not disclose an electrolyte layer containing a redox active material and thus requires rather large voltage applications which are deleterious to the long term stability of an EC device.
An object of this invention is to provide improved electrochromic devices, particularly relatively large area devices such as architectural and automotive glazings, which are stable, durable and have an enhanced memory.